Ultra HDR is a true HDR image format, and is backcompatible. libultrahdr is the reference codec for the Ultra HDR format. The codecs that support the format can render the HDR intent of the image on HDR displays; other codecs can still decode and display the SDR intent of the image.
After a recent update ultrahdr_app api mode 4 appears to be broken again, seemingly related to issue https://github.com/google/libultrahdr/issues/157
ultrahdr_app returns the error "did not receive valid image width for encoding. width : 0" but does not accept width or height parameters as input.